Various designs of motor-driven screwing and transporting tools for tightening or loosening the head bolting of pressure vessels are known. Such tools are known particularly for nuclear reactor pressure vessels, because in the case of nuclear reactor pressure vessels the problem arises to remove and then replace and tighten the head of the reactor pressure vessel quickly and safely, to minimize the radiation exposure of the personnel as much as possible during refueling. In nuclear reactor plants the shutdown time for refueling is influenced very substantially also by the time required for opening and closing the reactor pressure vessel. Here, the pressure vessel head is firmly bolted to the vessel proper via threaded studs which are sunk into the flange of the vessel, and associated tightening nuts on the upper side of the head.
A motor-driven screwing and transporting tool for loosening, transporting and tightening of individual tightening nuts of threaded studs is already known (German Offenlegungsschrift No. 2,061,037). As the loosening of individual closing or tightening nuts, the transporting and retightening in a definite order (to avoid asymmetrical stresses of the head and the pressure vessel) still requires relatively much time, thought has been given to the problem of pretensioning simultaneously several studs arranged in a pitch circle, to turn the associated vessel nuts up or down and to transport them forward and away. Thus, one known screw-tightening device (German Offenlegungsschrift No. 2,243,045) comprises the following elements: A support body which can be placed on the head flange, adjustably in height; tightening and loosening units mounted at the support body; means at the studs and their tightening nuts for coupling the tightening and loosening units and specifically, external teeth at the tightening nuts for the engagement of the drive pinion of a nut-rotating drive of the tightening and loosening units, and teeth arranged at the head end of the threaded stud for engagement of a gripper unit, wherein the tightening and loosening units comprise in turn the following components: A hydraulic tensioning cylinder, supported at the support body, for axially elongating the studs by axial action on the gripping units via a power piston during the loosening or tightening of the tightening nuts: a drive for coupling and decoupling the gripper body relative to the studs as well as said nut-turning drive with a drive pinion for turning the tightening nut in the direction of screwing it on or off via its external gearing.
Beyond this, it has also been proposed already to improve a motor-driven screwing and transporting tool of the kind described by loosening or tightening, as far as possible, all the tightening nuts of a head bolting, as well as transporting them to and fro, simultaneously. In this older arrangement, hydraulic plungers are already attached at the screwing and transporting tool, by means of which the distance of the tool from the surface of the head can be adjusted in accordance with the progress of the unscrewing and screwing operation.